1. Technical Field
The present invention relates to magnetic recording medium of the vertical magnetization type.
2. Prior Art
As a means of magnetic recording having superior characteristics for very short wavelength recording, vertical magnetization has recently become more and more important. In such a recording method, a recording medium having an axis of easy magnetization which is vertical to the surface of the film medium is required. When a signal is recorded on such a recording medium, remanent magnetization becomes perpendicular to the recording medium surface, accordingly the demagnetization field within the recording medium decreases as the wavelength of the signal becomes shorter, and a superior signal reproduction is obtainable. vertical magnetization recording media hitherto used are manufactured by sputtering a magnetization film having an axis of easy magnetization perpendicular to the film surface directly on, or with a soft magnetic thin film such as permalloy as interface on, a non-magnetic substrate. For the vertical magnetization recording medium, a film mainly consisting of a Co-Cr film of below 30 wt% of Cr has a haxagonal close packed structure and its C-axis can be disposed perpendicularly to the film surface. It is possible to lower the saturation magnetization to such level that the anisotropy magnetic field of the vertical direction exceeds the demagnetization field, and accordingly the vertical magnetization recording medium can be realized.
However, when the sputtering method is used, the speed of forming the magnetization thin film is slow and accordingly it is difficult to produce a vertical magnetization recording medium with a low manufacturing cost. On the contrary, according to a vacuum deposition method (including a method such as an ion-plating method in which a part of evaporated atoms are ionized), the Co-Cr vertical magnetization recording medium can be formed at a high rate of several thousands of .ANG./sec. Though a vapor deposition of soft magnetic material such as permalloy may be considered, when a magnetic film is formed on a substrate with such material, the vertical magnetization recording medium is not obtainable. In the vapor deposition method, a high productivity is obtainable by utilizing a method of vapor deposition on a substrate running around a cylindrical can.
FIG. 1 shows such vapor deposition system, wherein in an evacuated enclosure 100, a substrate, for instance, high polymer plastic film 1 is driven around a cylindrical can 2 which is kept at a temperature of about 200.degree. C. and rotates around a shaft S, and is fed from a feeding roll 3 to a winding roll 4. A deposition source 6 is heated to a high temperature and the source material is deposited through a slit 7 of a mask 5 onto the running substrate 1.
In order to produce the vertical magnetization recording medium of the Co-Cr deposition film, the C-axis of the hexagonal close packed structure must be disposed vertically to the film surface, and the anisotropic magnetic field in the vertical direction must be larger than the demagnetization field. In other words, the vertical anisotropic constant Ku must be a positive value. In order to fulfill the above-mentioned conditions, it has been experimentally confirmed that residue gas in the vapor deposition should be decreased as small as possible. However, when a high polymer plastic substrate is used, due to an outgassing from the substrate during the vapor deposition, it is difficult to obtain a Co-Cr vertical magnetization recording medium of good characteristics, making the constant Ku a negative value. Though the high polymer substrate can be outgassed by leaving in a high temperature low humidity atmosphere, the substrate part inside the thick roll can not be outgassed. Accordingly, special apparatus becomes necessary in order to produce a vertical magnetization recording medium of Co-Cr with good characteristics by eliminating the adverse influence of the residual gas.